The present invention relates to a split injector liner for a vapor sampling system, such as an equilibration headspace extraction system, and more particularly to such a split injector liner which can be used to interface a vapor sampling system, via a conventional gas chromatographic split injector, to a capillary column so that there is no dilution or dispersion of the sample vapor and so that optimum chromatographic separation and detection limits are preserved.
Certain sample extraction systems produce a homogenous sample vapor from constituent volatile organic compounds (VOCs) that can be analyzed by gas chromatography (GC). One such system is a headspace sampler whereby a liquid or solid sample is sealed in a suitable vial and placed inside a thermostatted environment. The VOCs within the sample become distributed between the sample matrix and the gaseous headspace above it and, in time, the concentration of sample vapor within the headspace reaches equilibrium. The vial is then pressurized with an inert carrier gas and a plug of the headspace vapor is allowed to elute from the vial and be introduced into a gas chromatograph where the composition of the headspace vapor is determined. The concentration of each compound within the original sample matrix is calculated by applying appropriate calibration response factors.
Many applications using such techniques require the chromatographic separation of many extracted compounds and their detection at very low levels. It is, therefore, critical to achieve an efficient transfer of the plug of the sample headspace vapor from the vial to the chromatographic column without dilution or dispersion. Any such effect would degrade component detection limits and chromatographic separation respectively.
One of two approaches is normally adopted for the interface between the vial and the chromatographic column: splitless transfer or split transfer.
In a splitless transfer interface, the chromatographic column may be effectively connected directly to the pressurized sample vial. The sample may then elute into the column without the need for additional carrier gas. This approach is termed xe2x80x9cPressure Balanced Samplingxe2x80x9d.
One of the problems with splitless transfer is that the flow rate from the vial to the column is very lowxe2x80x94with narrow-bore capillary columns this may be less than 0.5 ml/min. At such low flow rates, the internal capacity of the sampling mechanism and connecting lines is significant and so a major portion of the sample plug entering the column comprises residual carrier gas from these internal voids. This in effect causes sample vapor dilution and dispersion and compromises chromatographic performance. For this reason, wider sample vapor plugs are normally selected during headspace sampling. However, although this gives better detection limits, it does reduce chromatographic separation and precludes the use of narrow-bore columns.
Moreover, some systems use a sampling valve between the vial and the column. However, this requires additional carrier gas to transfer the sample vapor from the valve to the column, and the flow rate is still low and internal voids are still present.
In a split transfer interface, a splitter may be introduced into the path between the headspace vial and the chromatographic column in order to reduce the effect of any internal voids in the sample path. This splitter allows a moderate flow rate (typically 20 ml/min) of the sample vapor to be vented from the system. This has the effect of increasing the total volumetric flow through the vial-column interface and so reduces the detrimental effects of the internal voids on the resultant chromatography.
For user convenience and flexibility, it is attractive to utilize a standard split injector, as normally used for injection of liquid samples, for the headspace splitter. However, the use of an injector in such a manner requires the use of additional carrier gas to be supplied to the injector. While this allows the control of carrier gas pressure through the column independently of the conditions applied at the sample vial, it causes significant dilution of the sample vapor before it enters the column.
Therefore, neither the splitless nor split interface approaches described above is fully able to deliver a narrow, undiluted plug of sample from the headspace vial into a narrow-bore GC capillary column.
What is desired, therefore, is a split injector liner which can be used to interface a vapor sampling system, via a conventional gas chromatographic split injector, to a capillary column so that there is no dilution or dispersion of the sample vapor and so that optimum chromatographic separation and detection limits are preserved.
Accordingly, it is an object of the present invention to provide a split injector liner which can be used with a standard GC split injector.
Another object of the present invention is to provide a split injector liner having the above characteristics and which enables the delivery of a narrow, undiluted plug of sample vapor from a pressurized headspace vial into a narrow-bore GC capillary column.
A further object of the present invention is to provide a split injector liner having the above characteristics and which improves analytical detection limits as compared to known systems.
Still another object of the present invention is to provide a split injector liner having the above characteristics and which enables faster analysis times as compared to known systems.
Yet a further object of the present invention is to provide a split injector liner having the above characteristics and which can be employed with many known GC sampling systems, such as thermal desorption, purge and trap and gas sampling valves, where sample vapor must be transferred efficiently to a GC column.
These and other objects of the present invention are achieved by provision of a gas chromatography split injector including a housing having a carrier gas inlet port and a vent port and an injector liner. The injector liner includes a substantially cylindrical outer liner member disposed within the housing and an inner liner member disposed within the outer liner member. The inner liner member has substantially cylindrical upper and lower portions and a constriction therebetween. The constriction includes an inner surface having an inner diameter smaller than a diameter of inner surfaces of the upper and lower portions. The injector also includes a chromatographic column passing through the lower portion of the inner liner member and is in sealing engagement with the constriction of the inner liner member. A transfer line passing through the upper portion of the inner liner member and has an end located proximate to the constriction of the inner liner member. A sealing member is disposed between the outer liner member and the housing at a position between the carrier gas inlet port and the vent port.
A first fluid passageway is defined by an inner surface of the inner liner member such,that the transfer line is in fluid communication with the chromatographic column therethrough. A second fluid passageway is defined by an outer surface of the transfer line and an inner surface of the inner liner member, an outer surface of the inner liner member and an inner surface of the outer liner member, and an outer surface of the outer liner member and an inner surface of the housing such that the transfer line is in fluid communication with the vent port therethrough. A third fluid passageway is defined by an outer surface of the inner liner member and an inner surface of the outer liner member, and an outer surface of the outer liner member and an inner surface of the housing such that the carrier gas inlet port is in fluid communication with the vent port therethrough.
Preferably, the split injector also includes a first pressurized gas supply supplying pressurized gas to the transfer line and a second pressurized gas supply supplying pressurized gas to the carrier gas inlet port. Most preferably, the first pressurized gas supply supplies gas at a higher pressure than the second pressurized gas supply. Also, most preferably, the end of the transfer line is positioned about 2 millimeters away from the constriction of the inner liner member.
The inner liner member preferably includes an outwardly flaring flange portion extending from an upper edge of the upper portion to facilitate insertion of the transfer line therethrough, the flange portion having an outer diameter greater than a diameter of the inner surface of the outer liner member. It is also preferable for the outer liner member to include a reduced diameter channel portion at a lower end thereof, the reduced diameter channel portion having an inner diameter smaller than an outer diameter of the lower portion of the inner liner member. Most preferably, a lower edge of the lower portion of the inner liner member is beveled such as to inhibit relative sealing between the inner liner member and the outer liner member, and a lower edge of the reduced diameter channel portion of the outer liner member includes an inwardly flaring frustoconical portion to facilitate insertion of the chromatographic column therethrough.
In another aspect, the invention comprises an injector liner which can be used with a standard gas chromatography split injector so as to create a split injector as described above.
The invention and its particular features and advantages will become more apparent from the following detailed description considered with reference to the accompanying drawings.